Application Note- 900

Power Supply Functional Testing

A DC power supply is a device which transfers AC main power to a required DC output V/A/W rating. A good power supply should be reliable, meet all required functional specifications, full protection features, safety and Electromagnetic compatibility requirements. This application note is focusing on the testing of functional specifications and protection features.

Power supply testing methods for design, production, and quality verification require sophisticated electronic equipment. Different power supply configurations and output combinations also dictate the need for versatile test instruments that can accommodate a broad range of specifications.

Prodigit 3300 series Mainframe, 3310/3320 series Electronic Load Module, and 3600A power supply testers provides programmability, friendly operation, reliable test results, efficient testing and high testing quality at a minimal testing cost. Those instruments have been broad used to test the function of power supply by known manufacturer worldwide.

Following are functions typically tested when qualifying a switching power supply.

1. Function test

Hold on adjust
Line Regulation
Load Regulation
Combined Regulation
Ripple and Noise or PARD.
Input Power and Efficiency
Dynamic load or Transient load
Power Good/Power Fail (Power Good Signal)
Set-up/Hold-up time

Protection test:

Over Voltage Protection (OVP)
Over Current Protection (OCP)
Short Circuit Protection

1.1 Hold on adjust/set output voltage

When manufacturing switching power supplies, the first test step is to adjust output voltage to within a specified range. This is done first to ensure further specifications are met. Normally, the AC line voltage is set to nominal and the Dc output current is set to a nominal or a maximum load current in the HOLD-ON adjust procedure. The DVM measures the power supply's output voltage, and adjusts the potentiometer until the voltage reading is within the required limit.

1.2 Line Regulation

Line regulation is defined as a power supply's ability to provide a stable output voltage under conditions of changing input line voltage.

To accurately measure line regulation, the following
equipment is required:
a. A variable line source capable of providing at least the minimum to maximum input range of the power supply to be tested.
b. A true RMS voltmeter to monitor the input source voltage.
c. A precision DC voltmeter with an accuracy at least 10 times better than the regulation of the unit under test.
d. A variable load for output.

Typically, the equipment is set up as shown below:

During testing, allows the power supply unit under test to warm up and stabilize with a normal input voltage and load, Output voltage reading should then be taken with low, normal and high line input. Output voltage reading deviation from normal to low and high line conditions generate the Line regulation quality for that load condition. Line regulation is normally specified as a percentage of deviation from nominal output at a fixed load and is calculated using the following equation.

Line regulation can also be specified as the absolute DC output deviation within upper and lower voltage limit under changing input line voltage.

1.3 Load Regulation

Load regulation is a power supply's ability to provide a stable output voltage under conditions of changing load. Equipment and set-up required are very similar to that for line regulation. The only change required is the connection of an additional precision Ammeter in series to the output as illustrated below:

During testing, allow the power supply under test to warm up and stabilize, then measure the output voltage and use it as normal output voltage (Vnormal). Additional output voltage reading are then taken with maximum (Vmin) and minimum (Vmax) loads on the output. Output voltage deviation from normal to full load and minimum load generate the Load regulation.

Load regulation is normally specified as a percentage of deviation from nominal output at a fixed input voltage and is calculated using the following equation:

Load regulation can also be specified as the absolute DC output reading within upper and lower voltage limit.

1.4 Combined Regulation

Combined regulation is a power supply's ability to provide a stable output voltage under conditions of changing line voltage and load current. It is a combination of line regulation and load regulation, and provides more exact verification of a power supply's DC output by changing line input and load output.

Combined regulation is specified as the absolute DC output deviation within upper and lower voltage limits under changing input line voltage and output load current.